Version 100 of Chrome, Firefox & Edge May Break Websites

Mark Porter

Posted 7 March, 2022 by in Screaming Frog SEO Spider

Version 100 of Chrome, Firefox & Edge May Break Websites

Google Chrome, Mozilla Firefox and Microsoft Edge are rapidly closing in on a big milestone: version 100. While this sounds like a cause for celebration, it could result in endless headaches for a small number of websites, due to the bugs and compatibility issues that come with a triple-digit user agent string.

Read on to find out why people are anticipating something akin to the Y2k bug all over again.

What Is a User-Agent?

To briefly summarise, a browser’s user-agent is a string (line of text) that helps identify which browser is being used, what version it is and what operating system is being used. It’s sent by the browser to the server through a HTTP header, and can be used for things like serving a mobile version of a site if the request is coming from a smartphone.

To give you an example, the current user-agents for Chrome, Firefox and Edge are:

  • Chrome: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.51 Safari/537.36
  • Firefox: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:96.0) Gecko/20100101 Firefox/96.0
  • Edge: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.51 Safari/537.36 Edg/98.0.1108.62

Why Might Version 100 of Chrome, Firefox and Edge Break Websites?

The major concern with version 100 is due to the fact that the version number will be moving from two-digit to three-digit. The web uses various ways of handling and parsing user-agent strings, and similar issues were experienced when browser versions moved from single-digit to double-digit a little over 12 years ago.

In Mozilla‘s own words:

“Without a single specification to follow, different browsers have different formats for the User-Agent string, and site-specific User-Agent parsing. It’s possible that some parsing libraries may have hard-coded assumptions or bugs that don’t take into account three-digit major version numbers. Many libraries improved the parsing logic when browsers moved to two-digit version numbers, so hitting the three-digit milestone is expected to cause fewer problems. Mike Taylor, an engineer on the Chrome team, has done a survey of common UA parsing libraries which didn’t uncover any issues. Running Chrome experiments in the field has surfaced some issues, which are being worked on.”

Both Mozilla and Google have been testing this for some time, and there are backup strategies in place should the level of disruption be higher than anticipated. Mozilla and Google have both stated that if there are widespread issues, they can temporarily freeze major versions to 99 to avoid further problems.

When Are Version 100 of Chrome, Firefox and Edge Releasing?

Version 100 of the browsers are set to release on the following dates:

  • Chrome: March 29, 2022
  • Firefox: May 3, 2022
  • Edge: Week of 31-Mar-2022

They do however go through experimental and beta versions before they are rolled out on the stable channel, so these dates may change.

What Websites are Impacted?

It’s anticipated that only a very small number of websites are impacted, but the list of broken websites is growing each day. Notable names include, and

Out of interest, we bulk crawled over 3,000 domains from the Majestic Million, and didn’t find any other instances of broken websites.

The sites that aren’t compatible returned ‘403 Forbidden’ response codes, which would result in users receiving a error page when using version 100 of Firefox, Chrome or Edge. However, some websites may redirect users to an ‘unsupported browser’ page, so be sure to check redirecting URLs and their destination too.

For example, Standard Chartered’s Indian site is currently redirecting to an unsupported browser page advising users to upgrade their browser.

Further to this, Googlebot also regularly updates it’s user-agent string to match the latest stable version of Chrome. This means that if version 100 is resulting in your site returning a 403 Forbidden response code and Googlebot update, you could experience crawling and indexing issues.

While it is a small number of websites in the grand scheme of things, you definitely don’t want to be one of them.

How to Check if Your Website Is Impacted

If you’re wanting to check websites at scale, for example a list of your clients, the Screaming Frog SEO Spider can check these in seconds.

To do so you’ll first want to set a custom user-agent by going to Configuration > User-Agent > Custom (dropdown). The user-agents you can use to simulate version 100 are:

Mozilla/5.0 (Windows NT 10.0; rv:100.0) Gecko/20100101 Firefox/100.0

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4650.4 Safari/537.36

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.51 Safari/537.36 Edg/100.0

Once the user-agent has been set, switch to list mode (Mode > List) and you upload sites in bulk from a file or by pasting them in.

You’ll want to try all the user-agents to be sure, as at the moment some websites may load fine with Chrome v100, but not Firefox v100 and vice versa.

A domain returning a 403 Forbidden response code is a cause for concern, as it could mean it’s not compatible version 100 of Firefox, Chrome and Edge. Double check this using the browser themselves (information on how in the next section), as it could just be some kind of protection blocking the Spider Tool, such as Cloudflare.

As mentioned previously, be sure to thoroughly check any domains that are 30X redirecting, as some websites will redirect users to an unsupported or out of date browser warning page. To follow redirects in list mode, use the ‘Always Follow Redirects’ option under ‘Config > Spider > Advanced’.

Check a Website Using a Browser

If you just need to check a small number of websites, you don’t need to use the SEO Spider. Both Firefox and Chrome allow you to report the major version as 100:

For Chrome:
Go to chrome://flags/#force-major-version-to-100
Set the dropdown to ‘Enabled’

For Firefox:
Download and install Firefox Nightly
Open the settings and search for ‘Firefox 100’, and then enable ‘Firefox 100 User-Agent String’


We highly recommend spending some time checking both your own and your client’s websites, to ensure they wont be impacted by the imminent arrival of version 100 of major browsers. The aforementioned Github page has lots more information on the nitty gritty behind this issue, and allows you to also flag websites that are experiencing problems.

Big shout out to John Mueller for getting this on our radar!

Mark is a self-confessed geek who has always had a love/hate relationship with blogging, coupled with an addiction to buying domain names. Outside of SEO, Mark loves Manchester United, gadgets and games. If anyone can beat him at FIFA he will happily buy them a drink.


  • Rob Kerry 2 years ago

    Just to clarify – Screwfix is blocking the fake User Agent / Browser, not breaking because of the 3 digit version number. If you visit the website after forcing Chrome’s version to 100, the website loads perfectly fine in real Chrome browsers (200 OK).

    • screamingfrog 2 years ago

      Hi Rob,

      Think it was only Firefox we saw the issue for Screwfix, which was reproducible in a browser.

      It worked in Nightly from memory, so perhaps some blocking of spoofing via Chrome.

      But take your point, there will be false positives.



  • editajo 2 years ago

    For those of us who are dedicated to web development, the continuous updating of Chrome and Edge is crazy, and not so much of Firefox. Many sites that at the time they are published and work perfectly have to be reviewed, even changed hosting, for these updates.


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